Inkjet printing has become widely known and is most often implemented using thermal inkjet technology. Such technology forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. The printer, itself, can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate hardware and software to position the medium and expel droplets so that a desired graphic is formed on the medium. A conventional print cartridge for an inkjet type printer comprises an ink containment device and an ink-expelling apparatus, commonly known as a printhead, which heats and expels ink droplets in a controlled fashion. Typically, the printhead is a laminate structure including a semiconductor or insulator base, a barrier material structure which is honeycombed with ink flow channels, and an orifice plate which is perforated with nozzles or orifices with diameters smaller than a human hair and arranged in a pattern which allows ink droplets to be expelled. In an inkjet printer the heating and expulsion mechanism consists of a plurality of heater resistors formed on the semiconductor or insulating substrate and associated with an ink chamber formed in the barrier layer and one of the orifices in the orifice plate. Each of the heater resistors is connected to the controlling mechanism of the printer such that each of the resistors may be independently energized to quickly vaporize to expel a droplet of ink.
Most currently available thermal inkjet printers utilize a print cartridge which has a relatively small printhead (approximately 5 mm.times.10 mm) adjacent the media to be printed upon. The cartridge also contains a volume of ink which is coupled to the printhead. The entire print cartridge, including the volume of ink, is caused to shuttle back and forth across the width of a page of medium, laying down a swath of printed ink as the cartridge is moved across the page. Once the cartridge reaches the end of its print line, the medium is advanced perpendicularly to the direction of shuttle and another swath of ink is printed across the page. Moving the mass of ink contained in the print cartridge across the page places a limit on the speed at which the page can be printed and also constrains the amount of ink which can be stored in a print cartridge.
One technique which reduces or eliminates the shuttling of the print cartridge back and forth across the whole page is to utilize a printhead which is at least as wide as the media upon which print is to be placed, i.e. a page-wide printhead. Such an apparatus would print one or more lines at one time as the media is advanced, line by line, in a direction perpendicular to the long axis of the page-wide printhead. One such page-wide printhead has been described in U.S. patent application Ser. No. 08/192,087 "Unit Printhead Assembly For Ink-Jet Printing" filed on behalf of Cowger et al. on Feb. 4, 1994. This page-wide printhead employs a plurality of substrate modules aligned across the long axis of the page-wide printhead to enable easy replacement should one of the modular printheads suffer a failure.
One inherent problem with conventional page-wide printheads is that of manufacturability and thermal stability across the width of a page. In printers designed for office or home use, the width of a page-wide printhead equals 22 cm or more. In order to print with acceptable print quality, a page-wide printhead may have approximately 4800 printing orifices extending along the long dimension of the pagewide printhead. Because these orifices are small and misregistration of one orifice to another creates objectionable degredations in the quality of printing, it is important that the orifices be assembled with exceptional dimensional care and that the dimensions are held relatively constant over variations in temperature. Adding further to the temperature instability is the use of several different materials in the assembly of a conventional page-wide printhead. The printhead body typically is manufactured from plastic or metallic materials, upon which silicon substrates containing the firing resistors are affixed. The substrates are interconnected with a polyimide or other flexible polymer material. Each of these materials has a different coefficient of thermal expansion which leads to unacceptable misregistration of nozzles with temperature changes. An improperly matched set of materials can lead to rapid failure of a page-wide printhead. U.S. patent application Ser. No. 08/375,754 "Kinematically Fixing Flex Circuit to PWA Printbar" filed on behalf of Hackleman on Jan. 20, 1995, addresses one technique of accounting for thermal expansion of various materials used in a page-wide printhead. Furthermore, U.S. patent application Ser. No. 08/516,270 "Pen Body Exhibiting Opposing Strain To Counter Thermal Inward Strain Adjacent Flex Circuit" filed on behalf of Cowger on Aug. 17, 1995, provides an example of a plastic printhead body which may be designed to compensate the difference in thermal expansion of the various materials used in its construction.
The foregoing improvements notwithstanding, a page-wide printhead including precision manufacturing tolerances, thermal stability, and low cost is a highly prized but yet unrealized goal of inkjet printer designers.